Timer controlled thermostats which alternately maintain a space temperature at one level for a predetermined time interval and then at another level for a predetermined time interval are well known. In the past timers employed for this purpose were usually spring driven escapement movements or synchronous motor driven gear trains. More recently, however, considerably more compact and more accurate electronic timers with constant frequency pacing elements are being employed. Such electronic timers usually operate on a D.C. power supply and consume a relatively small amount of energy and may be operated satisfactorily on a very small storage battery provided the battery is rechargeable and circuit means for maintaining a charge thereon is provided.
Due to increasing public awareness of the necessity of conserving energy, there is currently a growing demand for replacement of existing conventional space thermostats with timer-controlled space thermostats so that the rate of heating or cooling of a space during selected periods of a day may be cut back in order to conserve energy. Obviously, it is highly desirable to provide a continuous electrical power supply to the electrically operated timer so that the selected cut-back periods of heating or cooling may be accurately maintained.
Conventionally space thermostats are positioned remotely from the space conditioning apparatus which they control and from the usual commercial A.C. power source which operates the space conditioning apparatus. Also, space thermostats are conventionally connected in series with the remote power source and either directly or through remote relay means with the space conditioning apparatus. Further, any temperature limiting switches which may be required are also conventionally connected in series with the space thermostat. Because of this, the electrical power supply to the electrically operated timer of a replacement timer-controlled space thermostat would be interrupted by any temporary failure of the usual commercial power supply or by the opening of any temperature limiting switch if the timer were operated on the commercial power supply and, therefore, some other electrical power source is desirable to provide an uninterrupted electrical power supply for the timer.
To provide an uninterrupted electrical power supply for the timer a small rechargeable storage battery positioned within the thermostat casing has been employed, and means for charging the battery only when the thermostat contacts are open has been proposed. By providing circuit means within the thermostat casing for charging the battery, a replacement timer controlled thermostat could be installed by the average homeowner without the necessity of stringing additional wiring from the remote power source. In the proposed arrangement for charging the battery only when the thermostat contacts are open, the charging circuit is connected across the thermostat contacts and a charging potential is applied to the battery only when the thermostat contacts are open. When the thermostat contacts are closed in this arrangement, the charging circuit is effectively shorted out. See U.S. Pat. No. 3,948,441.
Under conditions of use in which the thermostat contacts are closed to effect operation of space conditioning apparatus for considerably longer periods than they are open, it is desirable to apply a charging potential to the battery during periods when the contacts are closed. Also, under conditions in which the relative lengths of the periods during which the thermostat contacts are closed and open change substantially and frequently due to climatic changes, it is desirable to constantly apply a charging potential to the battery whether the contacts are closed or open to insure that a charge on the small battery is maintained.